Sequential chemical vapor deposition

ABSTRACT

The present invention provides for sequential chemical vapor deposition by employing a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. A first reactant forms a monolayer on the part to be coated, while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer but is available to react with the monolayer. A pump removes the excess second reactant and reaction products completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatuses suited to thelow temperature deposition of solid thin films of one or more elementsby the technique of sequentially exposing the object being coated withchemically reactive gaseous species. It also describes a number ofapplications of films produced by such processes.

CVD Reactor Technology

Chemical vapor deposition (CVD) reactors have been used for decades todeposit solid thin films and typical applications are coating tools,manufacture of integrated circuits, and coating jewelry. A. Sherman,Chemical Vapor Deposition for Microelectronics, Noyes Publications, NewJersey, 1987. Up to the 1960's many CVD reactors operated by exposing aheated object or substrate to the steady flow of a chemically reactivegas or gases at either atmospheric or reduced pressures. Since, ingeneral, it has been desired to deposit films at as high a rate aspossible as well as at as low a temperature as practical, the gases usedto produce the film are extremely reactive (e.g., silane plus oxygen todeposit silicon dioxide). Then if the gases are allowed to mix for toolong a time period before impinging the substrate, gas phase reactionscan occur, and in extreme cases there can be gas phase nucleation andparticles formed rather than deposition of continuous films. At the sametime, the high rate of deposition and the reactive gases used makes itvery difficult to coat large area substrates uniformly. This results invery complex and expensive commercial CVD reactors. A furthercomplication with this method is that in some cases the films depositeddo not conformally coat non-uniform surfaces. This can be particularlydeleterious in the manufacture of integrated circuits.

In the 1960's it was realized that we could lower the temperaturerequired for thin film deposition at acceptable rates by creating a lowpressure glow discharge in the reactive gas mixture. The glow dischargeproduces many high energy electrons that partially decompose thereactive gases, and these gas fragments (radicals) are very reactivewhen they impinge on a surface even at moderate temperatures. Althoughusing a glow discharge allows lower temperature operation, commercialreactors are very complex and expensive, since uniform deposition overlarge area substrates is even more difficult due to the inherentnonuniformity of glow discharges and due to the added expense of complexhigh frequency power supplies. Also, this technique can often lead todegradation of the film conformality, due to the highly reactive natureof the radicals.

In the 1970's atomic layer epitaxy (ALE) was developed in Finland by T.Suntola and J. Anston. U.S. Pat. No. 4,058,430 describes how they grewsolid thin films on heated objects. This process involves exposing theheated surface to a first evaporated gaseous element, allowing amonolayer of the element to form on the surface, and then removing theexcess by evacuating the chamber with a vacuum pump. When a layer ofatoms or molecules one atom or molecule thick cover all or part of asurface; it is referred to as a monolayer. Next, a second evaporatedgaseous element is introduced into the reactor chamber. The first andsecond elements combine to produce a solid thin compound monolayer film.Once the compound film has been formed, the excess of the second elementis removed by again evacuating the chamber with the vacuum pump. Thedesired film thickness is built up by repeating the process cycle many(e.g., thousands) times.

An improvement to this technique was described in a later patent issuingin 1983 to T. Suntola, A. Paakala and S. Lindfors, U.S. Pat. No.4,389,973. Their films were grown from gaseous compounds rather thanevaporated elements so the process more closely resembles CVD. This wasrecognized to be especially advantageous when one component of thedesired film is a metal with low vapor pressure, since evaporation ofmetals is a difficult process to control. With this approach, films weredeposited by flow reactors similar to a conventional CVD reactor, wherethe excess of each gas is removed by flowing a purge gas through thereactor between each exposure cycle. This approach was limited to only afew films, depending on the available gaseous precursors, and all ofthese films were not as contamination free as desired. We will refer tothis process as sequential chemical vapor deposition.

An alternative approach to operating a sequential chemical vapordeposition reactor would be to operate a non-flow vacuum system wherethe excess gaseous compound of each sequence is removed by vacuum pumpsin a manner similar to the original Suntola 1977 process. H. Kumagai, K.Toyoda, M. Matsumoto and M. Obara, Comparative Study of Al₂ O₃ OpticalCrystalline Thin Films Grown by Vapor Combinations of Al(CH₃)₃ /N₂ O andAl(CH₃)₃ /H₂ O₂, Jpn. Appl. Phys. Vol. 32, 6137 (1993).

An early application of sequential chemical vapor deposition was fordeposition of polycrystalline ZnS thin films for use in electrochromicflat panel displays. M. Leskela, Atomic Layer Epitaxy in the Growth ofPolycrystalline and Amorphous Films, Acta Polytechnica Scandinvica,Chapter 195, 1990. Additional studies have shown that other commerciallyimportant solid films of different compounds, amorphous andpolycrystalline, can be deposited by this technique on large area glasssubstrates. Among these other films are sulfides (strontium sulfide,calcium sulfide), transition metal nitrides (titanium nitride) andoxides (indium tin oxide, titanium dioxide). Elsewhere, this techniquehas been developed as a means of depositing epitaxial layers of groupIII-V (gallium indium phosphide) and group II-VI (zinc selenide)semiconductors, as an alternative to the much more expensive molecularbeam epitaxy process.

To applicant's knowledge the only literature discussing sequentialchemical vapor deposition of elemental films are those that depositelemental semiconductors in group IVB such as silicon and germanium. Onesuch study, S. M. Bedair, Atomic Layer Epitaxy Deposition Process, J.Vac. Sci. Technol. B 12(1), 179 (1994) describes the deposition ofsilicon from dichlorosilane and atomic hydrogen produced by a hottungsten filament. By operating the process at 650° C. deposition ofepitaxial films are described. Deposition of diamond, tin and leadfilms, in addition to silicon and germanium by an extraction/exchangemethod in conjunction with a sequential processing scheme similar tosequential chemical vapor deposition has also been reported M. Yoder,U.S. Pat. No. 5,225,366. Also although some of the studies reported haveexplored processes that may be useful at moderate temperatures, mostrequire undesirably high substrate temperatures (˜300-600° C.) toachieve the desired sequential chemical vapor deposition growth of highquality films.

Conformal Films Deposited at Low Temperatures for Integrated CircuitManufacture

A continuing problem in the commercial manufacture of integratedcircuits is the achievement of conformal deposition of dielectric (e.g.,silicon dioxide, silicon nitride) or conducting (e.g., aluminum,titanium nitride) thin solid films over large area wafers (e.g., 12inches in diameter). A film is conformal when it exactly replicates theshape of the surface it is being deposited on.

In one recent paper by D. J. Ehrlich and J. Melngailis, FastRoom-Temperature Growth of SiO₂ Films by Molecular-layer Dosing, Appl.Phys. Lett. 58, 2675(1991) an attempt was reported of layer by layerdeposition of silicon dioxide from silicon tetrachloride and water.Although the films appear to be very conformal, there is no discussionof film quality or density, and it is likely that these films are porousmaking them unsuitable for thin film applications. In support of thisconclusion, we can refer to a recent study by J. F. Fan, K. Sugioka andK. Toyoda, Low-Temperature Growth of Thin Films of Al₂ O₃ withTrimethylaluminum and Hydrogen Peroxide, Mat. Res. Soc. Symp. Proc. 222,327 (1991). Here, aluminum oxide deposited at 150° C. was compared todeposition at room temperature. In this case, the room temperature filmsthickness reduced from 2270 Å to 1200 Å upon annealing at 150° C. for 15minutes confirming the high porosity of the film deposited at roomtemperature. Another recent attempt to deposit silicon dioxide bysequential chemical vapor deposition used silane and oxygen by M.Nakano, H. Sakaue, H. Kawamoto, A. Nagata and M. Hirose, DigitalChemical Vapor Deposition of SiO₂, Appl. Phys. Lett. 57, 1096 (1990).Although these films, deposited at 300° C., appeared to be of betterquality, they were not perfectly conformal, and could only fill holes ofan aspect ratio up to 3:1. Modern integrated circuit technology requiresthe ability to coat holes and trenches with aspect ratios well in excessof 3:1.

Another technologically important thin solid film that needs to bedeposited with high purity and at low temperature, conformally overlarge area wafers, is the multilayer film of titanium and/or titaniumsilicide plus titanium nitride. Here, the need is for a thin titaniumand/or titanium silicide layer to be deposited on a silicon contact (˜10Å) followed by a layer of titanium nitride (˜3-400 Å). In a recent paperby K. Hiramatsu, H. Ohnishi, T. Takahama and K. Yamanishi, Formation ofTiN Films with Low Cl Concentration by Pulsed Plasma Chemical VaporDeposition, J. Vac. Sci. Techn. A14(3), 1037 (1996), the authors showthat an alternating sequence process can deposit titanium nitride filmsat 200° C. from titanium tetrachloride and hydrogen and nitrogen.However, the chlorine content of the films was ˜1%, and no attempt wasmade to deposit pure titanium metal or titanium silicide. Also, thereactor used was very similar to the conventional expensive plasmaenhanced CVD reactor.

Finally, sputtered aluminum films have been widely used to fabricateintegrated circuits for many years. Unfortunately, sputtering is a lineof sight deposition technique, so the films tend to be non-conformal.This has become more of a problem, in recent years, as denser circuitdesigns have resulted in holes of high aspect ratio that need to befilled. For this reason, many attempts have been made to find a suitablechemical vapor deposition process that would be highly conformal, andseveral processes have been successfully demonstrated by R. A. Levy andM. L. Green, Low Pressure Chemical Vapor Deposition of Tungsten andAluminum for VLSI Applications, J. Electrochem. Soc. Vol. 134, 37C(1987). Although conformal thin films of aluminum can be deposited byCVD, these films are still not acceptable for use in circuits, becausealuminum is susceptible to electromigration and it is necessary to addseveral percent of copper to these films to avoid this problem. All butone attempt to carry out the CVD process with copper precursors added tothe aluminum precursors have been unsuccessful. See E. Kondoh, Y.Kawano, N. Takeyasu and T. Ohta, Interconnection Formation by DopingChemical-Vapor-Deposition Aluminum with Copper Simultaneously: Al--CuCVD, J. Electrochem. Soc. Vol. 141, 3494 (1994). The problem is thatalthough there are CVD processes for the deposition of copper, theprecursors used interact with the aluminum precursors in the gas phasepreventing the simultaneous deposition of aluminum and copper.

Composite Fabrication

Many schemes have been developed to fabricate composite materials,because of the unusual strength of such materials. One approach to thefabrication of such materials is to prepare a cloth preform (i.e. fromthreads prepared from carbon fibers), and then expose this preform to ahydrocarbon gas at high temperatures. The hydrocarbon then pyrolyseswith carbon depositing on the carbon preform. Unfortunately, thisprocess is not very conformal, so that the outer pores of the preformare sealed before the interior can be coated, and the process has to bestopped prematurely. The preform then has to be machined to remove theouter layer, and further exposure is needed. This is a slow and veryexpensive process which is referred to in the literature as ChemicalVapor Infiltration (CVI); see e.g., Proceedings of the TwelfthInternational Symposium on Chemical Vapor Deposition 1993, eds. K. F.Jensen and G. W. Cullen, Proceedings Vol. 93-2, The ElectrochemicalSociety, Pennington, N.J.

Coating Aluminum with Aluminum Oxide

As is well known, coating aluminum with a thin layer of oxide is anexcellent way to protect this material from corrosion by the elements.The traditional way of doing this is to anodize the aluminum with a wetelectrochemical process (Corrosion of Aluminum and Aluminum Alloys, Vol.13 of Metals Handbook, ASM, Metals Park, Ohio, 1989). Pinholes and otherdefects in the anodized layer are the source of local failure of thecorrosion protection of the anodized layer. Such pinholes occur becausethe wet anodization process relies on the underlying aluminum as thesource of the aluminum in the aluminum oxide coating, and the underlyingaluminum can have many impurities and defects. A preferred approachwould be to deposit the desired aluminum oxide from an external source.Although using a CVD process to carry this out is an obvious choice,this has not been explored because the traditional CVD process operatesat ˜1000° C., and this far exceeds the melting point of the underlyingaluminum.

Low Temperature Brazing

In the manufacture of high temperature, high density ceramics, there isgreat difficulty in fabricating unusual shapes to high accuracy. Mostoften the ceramic is formed in the "green" state, machined while stillsoft, and then fired at high temperature. After firing, the resultinghigh density ceramic part may require additional machining, for example,with diamond grinding wheels, to achieve the desired dimensionalaccuracy. In some cases, the part shape makes this additional machiningdifficult and expensive, and in some instances there may be no know wayto reach the surface that needs to be ground. High temperature brazingof ceramic parts is an alternate technology for joining odd shapes ofaccurately finished ceramics. In some instances the braze metal may notbe compatible with the desired application. Also the high temperaturenecessary for metal brazing makes it difficult to join parts ofdifferent thermal expansion coefficients. For example, it is notpossible to braze aluminum to alumina ceramic, because the traditionalbrazing temperature would be far higher than the melting point of thealuminum.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a reactor operated atlow pressure, a pump to remove excess reactants, and a line to introducegas into the reactor through a valve. In this embodiment, a firstreactant forms a monolayer on the part to be coated, while the secondreactant passes through a radical generator which partially decomposesor activates the second reactant into a gaseous radical before itimpinges on the monolayer. This second reactant does not necessarilyform a monolayer but is available to react with the monolayer. A pumpremoves the excess second reactant and reaction products completing theprocess cycle. The process cycle can be repeated to grow the desiredthickness of film.

Because the film can be deposited one molecular layer at a time, thefilm forming on the part tends to be conformal and have uniformthickness. The present invention may use inexpensive reactors that cancoat many parts simultaneously reducing costs. For the formation of athree-element film, an additional step introduces a third reactant inthe process cycle. A stable compound film of any number of elements canbe formed by growing the monolayers of the elements with gaseousprecursors that contain the elements. Such precursors can be halides ororganometallic compounds.

It is an object of the invention to facilitate the growth of thin filmsof any element by using a radical generator to make available highlyreactive gases (radicals).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a sequential CVD reactor, suitable forthe deposition of films that are not electrically conducting,constructed in accordance with one embodiment of the present invention.

FIG. 2 illustrates a process cycle for the sequential CVD process.

FIG. 3 is a schematic drawing of a sequential CVD reactor, suitable forthe deposition of any film, conducting or non-conducting, constructed inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-section view of a reactor vessel 2 made of anon-conducting dielectric ceramic (e.g. a quartz cylinder) which issuitable for the deposition of a non-electrically conducting film on anon-electrically conducting part. The reactor vessel 2 forms a chamberclosed at one end by a flange 8, through which gases are introduced, andclosed at the other end by a flange 4 which connects to a vacuum pump 38through a pneumatically operated solenoid gate valve 36. Each flange hasan O-ring seal 6 to allow vacuum operation. The part 12 is placed in thereactor vessel 2 on a non-electrically conducting part holder 10. Avacuum gage 26 monitors the chamber pressure during operation. A firstreactant 28 is introduced as a gas into the chamber by evaporating aliquid or solid contained in bottle 30 by temperature controller 32 toprovide adequate vapor pressure for delivery into the chamber. In manysituations, the temperature controller 32 will provide heat to the firstreactant in the bottle 30. However, in others the controller may coolthe first reactant 28 in the bottle 30.

The first reactant 28 will be a compound having the elements of themonolayer to be formed on the part 12 such as the first reactants listedbelow in Examples 1-7. The first reactant 28 is introduced into thereactor vessel 2 through solenoid operated pneumatic valve 20 by amanifold 18. FIG. 1 illustrates a system with two bottles 30 and 31,each containing a first reactant 28 and 29, however, the type of film tobe formed will determine the number of reactants and bottles. Forexample, if a ternary film is desired, the system will include threebottles and three valves. A conventional digital microcontroller 40sequences the opening and closing of the valves 20 and 22 to deliver thefirst reactants to the chamber at the appropriate times as illustratedin FIG. 2.

Referring to FIG. 1, during a typical operation, a monolayer of thefirst reactant is deposited on the part 12 to be coated by exposure tothe first reactant 28 in vapor phase from the bottle 30. This monolayeris reacted by exposing it to a flux of radicals generated by the actionof a solenoidal coil 14, excited by a RF power supply 16, on moleculesintroduced from a gas bottle 34. The RF power supply 16 can becontrolled by the microcontroller circuit 40.

FIG. 2 illustrates a process cycle for forming thin films with reactorvessel shown in FIG. 1. Initially, the vacuum pump 38 evacuates thechamber of the reactor vessel 2. The exhaust gate valve 36 then closesand a valve 20 opens for a short period of time to deliver the firstreactant 28 to the reactor vessel 2 in a sufficient amount to form amonolayer of molecules on the part 12 to be coated. After the monolayeris formed, the reactor vessel 2 is again evacuated by the vacuum pump 38to remove excess first reactant. Next, a second reactant from bottle 34is delivered into the reactor vessel 2 for a short period of time whilea solenoidal coil 14 is excited by the RF power supply 16 generatingradicals. This step is carried out for a sufficient period of time tofully react the radicals with the first reactant monolayer. Finally, thereactor vessel 2 is evacuated again by the vacuum pump 38 ending thefirst cycle. The process cycle can then repeat to form the desiredthickness of the film.

If the film to be deposited is electrically conducting, reactor vessel 2will be coated by a conducting film which eventually shields out theexciting electric field provided by the solenoidal coil 14. To avoidunnecessary reactor vessel cleaning, in another embodiment, the presentinvention provides the reactor vessel 3 as shown in FIG. 3. The exhaustflange 4 provides access to the interior of the reactor vessel 3. Theflow of second reactant 42 is generated in a radical generator 44 whichis attached to the wall of the reactor vessel 3. As before the firstreactant 28 is provided from the bottle 30 and introduced to the reactorvessel 3 through the valve 20 and the manifold 18. In this embodiment,the part holder 10 can be either a metal or a ceramic. Again themicrocontroller 40 controls all valves and the radical generator 44.

The radical generator 44, suitable for use with the reactor vessel 3,shown in FIG. 3, can take on many well known arrangements. Onearrangement is to use a miniaturized version of the quartz tube 2 and RFcoil 14 described in FIG. 1. In this arrangement, the only modificationis to provide an end plate with a small hole in it, so that the radicalscan flow rapidly into the reactor vessel 3 through such a nozzle. Oneillustration of a suitable end plate with a hole in it serving as anozzle is shown in FIG. 1, as a stainless steel anode, in a paper by A.Sherman, In situ Removal of Native Oxide from Silicon Wafers, J. Vac.Sci. Technol. Vol. B8(4), 656 (July/August 1990) which paper isincorporated by reference here in its entirety. This paper alsodescribes generating hydrogen radicals using a hollow cathode DCdischarge chamber. Other alternatives are reviewed for hydrogen radicalgeneration in a recent paper by V. M. Bermudez, Simple, EfficientTechnique for Exposing Surfaces to Hydrogen Atoms, J. Vac. Sci. Technol.Vol. A14, 2671 (1996). Similar techniques can be also used to generateany of the radicals that might be needed to form the elemental filmsdescribed herein.

Concerns about the uniformity of distribution of radicals should notcontrol the type of radical generator 44 to be employed. As long assufficient radicals are generated to react the first reactant, anyexcess radicals play no role in the film formation. More importantconsiderations relate to avoiding the introduction of contaminants, thecost of the radical generator, and simplicity of its operation. Also,the reaction between any one of the first reactants adsorbed on the partsurface and the radical flux to the part, should be rapid andindependent of surface temperature. Therefore, it should be possible tocarry out these thin film depositions at lower temperatures than inconventional sequential chemical vapor deposition processes which aretypically carried out at 300-600° C.

EXAMPLE 1

Deposition of thin films of silicon dioxide can be carried out with asilicon precursor, such as dichlorosilane which can be reduced toelemental silicon by a flux of hydrogen atoms. S. M. Bedair, AtomicLayer Epitaxy Deposition Process, J. Vac. Sci. Technol. B 12(1), 179(1994). It should also be possible to deposit elemental silicon fromother precursors (e.g., silane, tetramethylsilane) and atomic hydrogen.The resulting silicon can then be converted to silicon dioxide byexposure to oxygen. In this way a silicon dioxide film can be grownmonolayer by monolayer. Another way to grow this film would be to use asilicon precursor that already contains oxygen. For example, one coulduse tetraethoxysilane and reduce it with oxygen atoms.

EXAMPLE 2

In one embodiment, the present invention provides a process for coatinga part with an elemental metal film. For brevity sake, we will limit thediscussion to a titanium metal film. In this example, the first reactantcould be titanium tetrachloride which could be introduced into thereactor at a low pressure so that a monolayer adsorbs to the surface ofthe part. Next, any excess titanium tetrachloride in the reactor chamberis pumped out. In order to form pure titanium films, we could thenexpose the surface to low pressure hydrogen in atomic form. The hydrogenatoms will react with the chlorine in the titanium tetrachloridemonolayer to form HCl. The HCl vapor can then be exhausted by a vacuumpump, and a monolayer of titanium will be left behind. The thickness ofthe titanium metal film is determined simply by the number of processcycles carried out. By this process it is possible to grow a film of anyelement that is solid at room temperature.

Deposition of thin titanium plus titanium nitride compound films couldbe derived from titanium tetrachloride and hydrogen atoms to yield thepure titanium, followed by exposure to nitrogen atoms to form thenitride. Alternately, we could expose titanium tetrachloride to NHradicals to produce titanium nitride films directly. Again, if we wishto use a precursor that contains both the titanium and nitrogen atoms,such as tetrakis(diethylamino)titanium ortetrakis(dimethylamino)titanium, we could reduce a monolayer of eitherof these species with hydrogen atoms or HN radicals to form titaniumnitride.

EXAMPLE 3

The present invention provides for growing a film with three or moreelements such as an oxynitrides by sequentially growing an oxide andthen growing a nitride. In fact, there would be no difficulty in growingternary compounds such as tantalum/silicon/nitrogen which is a gooddiffusion barrier film for advanced integrated circuits.

Various binary and ternary silicides can be formed by depositing one, ormore, metallic or semiconductor elements and nitriding the layer withnitrogen atoms. For example, we could deposit a monolayer of puresilicon, and then a monolayer of pure titanium. If the resultingmonolayer of titanium silicide were then nitrided with a flux ofnitrogen atoms, we could have a titanium/silicon/nitrogen ternarycompound. Also, the stoichiometry of the compound film could be changedsimply by changing the number of cycles used for any of the elements.For example, titanium disilicide (TiSi₂) could be formed from twosilicon cycles for each titanium cycle.

EXAMPLE 4

Deposition of aluminum films doped with copper and silicon could beformed from triisobutylaluminum, copper(II)acetylacetonate Cu(acac)₂ !,and tetramethylsilane each reduced in turn by hydrogen atoms. Thepercentages of copper and/or silicon dopants could be adjusted bycontrolling how many layers of each element are deposited. For example,a two percent doping level of copper is achieved by depositing one layerof copper for every 50 layers of aluminum.

EXAMPLE 5

If we take full advantage of the ability of the sequential CVD processto conformally coat parts that are very porous, we could fabricate anumber of important composite materials. For example, we could grow acarbon layer from methane and hydrogen atoms. This layer could then beconverted to a silicon carbide by growing a silicon layer as describedin Example 1. This silicon carbide coating could be used to coat acarbon fiber preform until a solid silicon carbide body is formedreinforced with carbon fibers. The carbon fibers would give the partgreat strength, and the silicon carbide would allow it to be used athigh temperatures in air. Ceramic composites using alumina whiskerscould be formed by growing aluminum oxide on a preform made from suchfibers. Metallic composites could be also prepared using metallic fiberpreforms and a sequential CVD to grow metal on the preform.

EXAMPLE 6

We now know that good quality aluminum oxide thin films can be grown atmoderate temperatures by H. Kumagai, K. Toyoda, M. Matsumoto and M.Obara, Comparative Study of Al₂ O₃ Optical Crystalline Thin Films Grownby Vapor Combinations of Al(CH₃)₃ /N₂ O and Al(CH₃)₃ /H₂ O₂, Jpn. J.Appl. Phys. 32 6137 (1993) by sequential CVD. It is, therefore, possibleto coat anodized aluminum parts with this highly conformal layer. Theearlier CVD processes could not be used because they had to be operatedat temperatures higher than the melting point of aluminum. One approachwould be to use known methods of sequential CVD to coat aluminum. Analternative approach would be to take advantage of the process describedin the present invention, where we can form monolayers of pure aluminumand then oxidize these layers with oxygen atoms. For example, we couldreduce trimethylaluminum with hydrogen atoms to form the aluminum layer.This layer will readily oxidize when exposed to oxygen. If the aluminumwere initially anodized, the sequential chemical vapor deposition filmwill fill in any defects or pinholes.

EXAMPLE 7

Joining two pieces of ceramic at low temperature with a pure ceramicmaterial, is a process that has some unique advantages. For example, thetemperature tolerance of the joined parts will be as high as theoriginal ceramic parts. Also, no new material is added to the structure,so the resulting joined part is of high purity, and just as chemicallyinert as the original ceramics. Such a process does not exist today. Forexample, two pieces of aluminum oxide could be joined by growingaluminum oxide, as described in Example 6, on the two adjacent parts.

While the invention has been illustrated in particular with respect tospecific methods of carrying out the same, it is apparent thatvariations and modifications can be made. It will be apparent from thepreceding that the present invention significantly advances the state ofthe art in the technology of sequential chemical vapor deposition ofthin films, and describes several commercially significant applicationsfor films deposited by the method of this invention. The process of thisinvention is unique in that it allows, for the first time, thedeposition of perfectly conformal and very pure films of any compositionat low temperatures.

The commercial applications of the films deposited by the technique ofthis invention should not be limited to this method of creating thesefilms. In some instances, films grown by known sequential CVDtechniques, without resort to radicals may be adequate depending on theapplication.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in theform and details may be made therein without departing from the spiritor scope of the invention.

What is claimed is:
 1. A process of growing a thin film by a sequentialchemical vapor deposition process, comprising the steps of:placing apart in a chamber; evacuating the chamber of gases; exposing the part toa gaseous first reactant, including a non-semiconductor element of thethin film to be formed, wherein the first reactant adsorbs on the part;evacuating the chamber of gases; exposing the part, coated with thefirst reactant, to a gaseous second reactant of radicals, wherein theradicals convert the first reactant on the part to one or more elements,wherein a thin film is formed; and evacuating the chamber of gases. 2.The process of claim 1, wherein the parts to be coated are maintained attemperatures up to 500° C. in the chamber.
 3. The process of claim 1,wherein the first reactant includes a halogen atom.
 4. The process ofclaim 1, wherein the first reactant is a metal-organic moleculesincluding an elemental metal species.
 5. The process of claim 1, whereinthe radicals of the second reactant are selected from the groupconsisting of oxygen atoms, nitrogen atoms, hydrogen atoms, OHmolecules, NH molecules or a combination thereof.
 6. The process ofclaim 1, wherein the radicals of the second reactant are generated in aradical generator in communication with the chamber.
 7. The process ofclaim 1, wherein the radicals of the second reactant are generated bysurrounding the chamber with an RF excitation coil, wherein the thinfilm being formed is a dielectric material.
 8. The process of claim 1,wherein the first reactant is tetraethoxysilane and the second reactantis atomic oxygen which converts the first reactant to a thin film ofsilicon dioxide.
 9. The process of claim 1, wherein the first reactantis titanium tetrachloride and the second reactant is atomic hydrogenwhich converts the first reactant to a thin film of titanium.
 10. Theprocess of claim 1, wherein the first reactant is a titaniumorganometallic compound and the second reactant are hydrogen atoms or HNmolecules which convert the first reactant to a thin film of titaniumnitride.
 11. The process of claim 1, wherein the first reactant is analuminum compound and the second reactant is atomic hydrogen whichconverts the first reactant to a thin film of aluminum.
 12. The processof claim 1, wherein the first reactant is a copper compound and thesecond reactant is atomic hydrogen which converts the first reactant toa thin film of copper.
 13. A process of coating a fiber preform with aconformal thin film by a sequential chemical vapor deposition process toform a low porosity structure of high strength, comprising the stepsof:forming a porous material in a predetermined shape; inserting theporous material in a chamber; evacuating the chamber of gases; exposingthe porous material to a first reactant, including one or more elementsof the film to be formed, wherein the first reactant adsorbs on thepart; evacuating the chamber of gases; and exposing the porous materialto a second reactant which converts the first reactant to a thin film.14. A process of coating aluminum with aluminum oxide, formed by asequential chemical vapor deposition process, so that the coating ishighly conformal and provides protection against corrosion of thealuminum, comprising the steps of:inserting the aluminum part in achamber; evacuating the chamber of gases; exposing the aluminum to afirst reactant, including aluminum as an element, wherein the firstreactant adsorbs on the part; evacuating the chamber of gases; andexposing the aluminum coated with the first reactant to a secondreactant which oxidizes and converts the first reactant to a thin filmof aluminum oxide.
 15. A process of growing a thin layer of ceramicmaterial between adjacent parts to cause the parts to be joined for aceramic brazing process, comprising the steps of:inserting the ceramicparts in a chamber; evacuating the chamber of gases; exposing theceramic parts to a first reactant which adsorbs on the ceramic parts;evacuating the chamber of gases; and exposing the ceramic parts coatedwith the first reactant to a second reactant which converts the firstreactant to a thin film of ceramic material.
 16. A process of growing athin film by a sequential chemical vapor deposition process, comprisingthe steps of:placing a part in a chamber; evacuating the chamber ofgases; exposing the part to a gaseous first reactant, containing ametallic element, wherein the first reactant adsorbs on the part;evacuating the chamber of gases; exposing the part, coated with thefirst reactant, to a gaseous second reactant of radicals, wherein theradicals convert the first reactant on the part to a metallic element;and evacuating the chamber of gases.
 17. The process of claim 16,wherein the first reactant is a metal selected from a group consistingof aluminum, copper, tantulum, titanium, and tungsten, and the secondreactant is a radical selected from a group consisting of hydrogen,nitrogen, oxygen, or a combination thereof.
 18. A process of growing athin film by a sequential chemical vapor deposition process, whichcomprises:placing a part in a chamber; evacuating the chamber of gases;exposing the part to a gaseous first reactant selected from the groupconsisting of Si and Ge, wherein the first reactant adsorbs on the part;evacuating the chamber of gases; exposing the part, coated with thefirst reactant, to a gaseous second reactant of radicals selected fromthe group consisting of O and N, wherein the radicals convert the firstreactant on the part to a thin film selected from the group consistingof silicon nitride, silicon oxide, germanium nitride, germanium oxide;and evacuating the chamber of gases.
 19. A process of growing silicondioxide thin film by a sequential chemical vapor deposition process,which comprises:placing a part in a chamber; evacuating the chamber ofgases; introducing a first reactant of gaseous silicon compound notcontaining oxygen into the chamber to expose the part to the firstreactant, wherein the first reactant includes the silicon of the thinfilm to be formed, so that the first reactant adsorbs on the part;evacuating the chamber of gases; exposing the part, coated with thefirst reactant to hydrogen atoms, wherein the hydrogen atoms reduce thefirst reactant on the part to silicon; evacuating the chamber of gases;and exposing the part coated with silicon to oxygen, wherein the oxygenconverts the silicon on the part to silicon dioxide.
 20. A process ofgrowing silicon dioxide thin film by a sequential chemical vapordeposition process, which comprises:placing a part in a chamber;evacuating the chamber of gases; introducing a first reactant of gaseoussilicon precursor containing oxygen into the chamber to expose the partto the first reactant, wherein the first reactant includes the siliconand oxygen of the thin film to be formed, so the first reactant adsorbson the part; evacuating the chamber of gases; exposing the part coatedwith the first reactant to oxygen atoms, wherein the oxygen atomsconvert the first reactant on the part to silicon dioxide; andevacuating the chamber of gases.